Getting the radial profile

This step is simple in concept, and a pain in execution unless you've got a script that does it all.  Having worked on these halos for a while (check out my CV), I now have a number of these scripts.  Effectively, what one does is determine how many annuli one wants, and then determines the number of counts in each annulus (from the event file) and the effective area as well (from the exposure map).  Dividing the two gives the surface brightness in units of photons/cm^2/s/pixel, which can be converted into any other convenient units later.  Here are the commands I use to make the annulus file:

sherpa
rad = 10^[-0.3:3.0:0.03]
fp = fopen("annuli.dat","w");
() = fprintf(fp,"%f %f\n",0.0,rad[0]);
for (i=0;i
()=fclose(fp);

And then I just use a script I wrote, make_radprof_hrc.sl, to calculate the actual surface brightness:

make_radprof_hrc.sl hrc_evt2.fits srcfree.reg ../expmap 270.28383 -25.07775 annuli.dat

This script assumes that the exposure map is named hrc_expmap.fits.  The central Right Ascension and Declination are given in the command line as 270.28383 and -25.07775; these values were obtained from the event file itself, using ds9.  Note that this position is close to, but not exactly at, the simbad position for GX5-1 (270.284167, -25.079167).  The separation is 5.2'' - which is rather large for Chandra.

This deserves a closer look to see if there is some reason the positioning of the source would be incorrect.  Normally, Chandra positions are better than 1'', so a 5+'' offset is odd.  Now, it's possible simbad is wrong - remember that Chandra has the highest angular resolution of any X-ray satellite, and that GX5-1 is first and foremost an X-ray source.  Fortunately, simbad lists sources, and in particular lists this Ebisawa et al (2003) paper as the source.  Checking this paper, we find that they reference Liu et al (2001) as their source for the position, with the source position accuracy given as 3'' with a caveat that this is only an estimate.  However, Liu et al. reference the paper "Infrared observations of Galactic bulge X-ray sources" (Hertz & Grindlay 1984), which sounds as though it should have quite accurate positions.  Nonetheless, a quick scan of Hertz & Grindlay shows that they got their positions from Einstein observations, and the infrared observations in fact did not detect GX5-1 at all. 

Upshot: The Chandra position is almost certainly superior to the position listed in Simbad.  You can't trust everything on the web, after all...